1. Field of the Invention
This invention relates to a fuel metering control system for an internal combustion engine.
2. Description of the Prior Art
The PID control law is ordinarily used for fuel metering control for internal combustion engines. The control error between the desired value and the manipulated variable (control input) is multiplied by a P term (proportional term), an I term (integral term) and a D term (differential or derivative term) to obtain the feedback correction coefficient (feedback gain). In addition, it has recently been proposed to obtain the feedback correction coefficient by use of modern control theory or the like, as taught by Japanese Laid-Open Patent Application No. Hei 1(1989)-110,853. As the control response is relatively high in such cases, however, it may under some engine operating conditions become unstable owing to controlled variable fluctuation or oscillation, degrading the stability of control.
It has therefore been proposed, as in Japanese Laid-Open Patent Application No. Hei 4(1992)-209,940, to calculate a first feedback correction coefficient using modern control theory, calculate a second feedback correction coefficient whose control response is inferior to (or lesser than) that of the first feedback correction coefficient using the PI control law, and determine the controlled variable using the second feedback correction coefficient during engine deceleration, when combustion is unstable. For a similar reason, Japanese Laid-Open Patent Application No. Hei 5(1993)-52,140 proposes determining the controlled variable using a second feedback correction coefficient of inferior control response when the air/fuel ratio sensor is in the semi-activated state. In Japanese Patent Application No. Hei 6(1994)-66,594 (filed in the United States on Mar. 9, 1995 under the number of 08/401,430), for example, the assignee proposes a system for determining the quantity of fuel injection using an adaptive controller.
In fuel metering control, the supply of fuel is shut off during cruising and certain other operating conditions and, as shown in FIG. 16, it is controlled in an open-loop (O/L) fashion during the fuel cutoff period. Then when the fuel supply is resumed for obtaining a stoichiometric air/fuel ratio (14.7:1), for example, fuel is supplied based on the quantity of fuel injection determined in accordance with an empirically obtained characteristic. As a result, the true air/fuel ratio (A/F) jumps from the lean side to 14.7: 1. However, a certain amount of time is required for the supplied fuel to be combusted and for the combusted gas to reach the air/fuel ratio sensor. In addition, the air/fuel ratio sensor has a detection delay time. Because of this, the detected air/fuel ratio is not always the same as the true air/fuel ratio but, as shown by the broken line in FIG. 16, involves a relatively large error.
At this time, as soon as the high-control-response feedback correction coefficient (illustrated as KSTR in the figure) is determined based on a control law such as the adaptive control law proposed by the assignee, the adaptive controller determines the feedback correction coefficient KSTR so as to immediately eliminate the error between the desired value and the detected value. As this difference is caused by the sensor detection delay and the like, however, the detected value does not indicate the true air/fuel ratio. Since the adaptive controller nevertheless absorbs the relatively large difference all at one time, KSTR fluctuates widely as shown in FIG. 16, thereby also causing the controlled variable to fluctuate or oscillate and degrading the control stability.
The occurrence of this problem is not limited to that at the resumption of fuel supply following cutoff. It also arises at the time of resuming feedback control following full-load enrichment and of resuming stoichiometric air/fuel ratio control following lean-burn control. It also occurs when switching from perturbation control in which the desired air/fuel ratio is deliberately fluctuated to control using a fixed desired air/fuel ratio. In other words, the problem arises whenever a large variation occurs in the desired air/fuel ratio. None of the aforesaid prior art references offer any measure for overcoming this problem.
It is therefore preferable to determine one feedback correction coefficient of high control response using a control law such as the adaptive control law and another feedback correction coefficient of low control response using a control law such as the PID control law (illustrated as KLAF in the figure) and to select one or the other of the feedback correction coefficients depending on the engine operating condition. Since the different types of control laws have different characteristics, however, a sharp difference in level may arise between the two correction coefficients. Because of this, switching between the correction coefficients is liable to destabilize the controlled variable and degrade the control stability.
A first object of the invention is therefore to provide a fuel metering control system for an internal combustion engine which determines feedback correction coefficients different in control response using multiple types of control laws and which smooths the switching between the feedback correction coefficients, thereby improving fuel metering and air/fuel ratio controllability while ensuring control stability.
The aforesaid level difference between the correction coefficients is particularly pronounced at the time of switching from the feedback correction coefficient of low control response to the feedback correction coefficient of high-response.
A second object of the invention is therefore to provide a fuel metering control system for an internal combustion engine which determines feedback correction coefficients different in control response using multiple types of control laws, which selects one thereof in accordance with the engine operating condition, and particularly which smooths the switching from the feedback correction coefficient of low control response to the feedback correction coefficient of high control response, thereby improving fuel metering and air/fuel ratio controllability while ensuring control stability.
As mentioned earlier, the problem is apt to arise at the resumption of the feedback control following open-loop control due to fuel supply cutoff, full-load enrichment or EGR operation, etc.
A third object of the invention is therefore to provide a fuel metering control system for an internal combustion engine which determines feedback correction coefficients different in control response using multiple types of control laws, which selects one thereof in accordance with the engine operating condition, and particularly which smooths the switching when resuming the feedback control upon returning to an open-loop control implemented at fuel supply cutoff, full-load enrichment or EGR operation, etc., thereby maintaining optimum balance between control stability and control convergence.
Furthermore, calculation of a manipulated variable using the adaptive control law proposed by the assignee earlier can enhance control accuracy and eliminate any difference between a desired value and a detected value all at one time. Although this provides control with excellent convergence, the feedback correction coefficient calculated by the adaptive control law may become unstable under certain engine operating conditions. Use of the feedback correction coefficient without modification may therefore not always enhance control stability.
A fourth object of the invention is therefore to provide a fuel metering control system for an internal combustion engine which determines a feedback correction coefficient of high control response using the adaptive control law or some similar laws, and when the feedback correction coefficient fluctuates, which can continue the feedback control while implementing an effective measure for maintaining optimum balance between control stability and control convergence.